Two known types of procedures for the fermentation of biogenic wastes are known, namely the widespread, batch-processing method with vertical tanks which are extremely slow but easily controlled, and the less widespread procedure, operating continuously in a regenerative flow with a horizontal fermentor which is however difficult to monitor and control.
In the case of the batch-processing method, one or more standing, vertical fermentors are used. The degree of fineness of the chopped biogenic waste is not of particular importance, but merely exerts an influence upon the duration of fermentation. The biogenically decomposed portions drift in the direction of the fermentor outlet. Only near the outlet are samples taken and the time determined at which a batch is to be discharged and new fresh material added. The problem in this case is not so much control, but rather that the heavier components migrate more quickly and consequently arrive at the outlet still not completely decomposed. This is all the more serious because they frequently contain pathogens not killed as a result of too brief a time in the fermentor.
To counteract this, fermentation plants are being increasingly built today with inclined fermentors. According to the degree of slant, the material will flow correspondingly more slowly. A rapid migration of relatively heavy components is thus largely prevented.
The requirement for all known anaerobic fermentors in fermentation plants is still however a mixture with relatively good capacity to flow in the fermentor, to achieve extensive homogeneity in the fermentor, because there are no possibilities available for making corrections.
The plant disclosed in DE-A 3,228,895 thus works with an inclined fermentor, itself rotatably mounted, which works with a mixture of clearing sludge and garbage. This produces a content with the capacity to flow relatively well, whose throughput rate can be controlled at the outlet side and can in addition be influenced by the angle of slant. The material to be loaded in can be given merely a preliminary check and its pH adjusted by adding lime.
The fermentor disclosed in DE-A 2,535,756, on the other hand, operates with highly fluid material, and a powerful agitator prevents the formation of surface scum. In this case, the entire content is mixed, which is not however problematic because batch processing is used.
But even with vertical fermentors the filling material is kept as fluid as possible to achieve homogeneous conditions in the ferment-or. Thus it is proposed in EP 131,319 that the fermentor be charged each time with one part fresh material and two parts of material which is already decomposed. In DE-A 2,924,387, on the other hand, the fill is chopped until the material is fine enough to be pumped.
Although mention is made of the regenerative flow process in various patents with inclined fermentors, it is really only a quasi regenerative flow process which takes place in spurts.